As a semiconductor silicon single-crystal growth method, the Czochralski method (hereinafter, the CZ method) is known. In this method, a seed crystal is immersed in a melt and is slowly pulled upwardly while being rotated, whereby a single crystal is grown. The single crystal is produced so as to have a certain diameter. For example, when a finished product is an 8 inch (200 mm) wafer, it is common to produce a crystal having a diameter of 200 to 210 mm, which is slightly greater than the diameter of the wafer. Then, the crystal is ground at the outer circumference thereof so as to have a cylindrical shape, and is sliced into wafers. The wafer is then subjected to chamfering, and has an ultimate target wafer diameter. The target diameter in the production of a single crystal has to be greater than the wafer diameter of a finished product. However, too large a diameter increases grinding/polishing stock removal, making it uneconomical. Therefore, there is a demand for a single crystal having a diameter, which is greater than that of a wafer and is as small as possible.
In the CZ method, there are mainly two methods for controlling a diameter, of which one is an optical method (a camera method) and the other is a weight method (a load cell method). In the optical method, a growing crystal in a furnace is observed through quartz glass by a camera attached outside the furnace. The position of the edge of the crystal is determined by processing an image taken by the camera, and the position thus determined is converted into a diameter by turning the position into coordinates. Moreover, the optical methods include a method for measuring the ends of a crystal, a method for measuring one side of a crystal, a method for determining a diameter based on the curvature of a circular arc, and the like.
However, with the method for measuring the ends of a crystal by a camera, it becomes difficult to capture a diameter D shown in FIG. 1 in its entirety as the diameter of the crystal increases. Moreover, the problem is that, even when the diameter is captured in its entirety, degradation in resolution occurs. Furthermore, as disclosed in JP-A-2004-35352, for example, there is a method by which two cameras are provided in a pulling apparatus and the ends are observed by using these cameras. However, in this case, an error caused by displacement of the relative positions of the cameras becomes a problem.
Moreover, as a method for measuring one side of a crystal, there is a method for determining a diameter based on a distance R from a virtual center point as shown in FIG. 2. However, a measurement error occurs due to displacement of the virtual point caused by displacement of the position of a camera.
Furthermore, the optical methods include a method for determining a diameter by calculating a distance R from a center point based on the curvature of a circular arc as shown in FIG. 3. However, the problem of this method is that the curvature becomes small as the diameter of a crystal increases, resulting in an increase in measurement error.
As described above, the problem of the single crystal diameter detection method using the optical method is that a measurement error occurs due to an increase in diameter of a crystal or displacement of a camera for detection. For example, when the crystal diameter deviates from a target, a problem arises, such as a reduction in yields due to product failure caused by an insufficient diameter and an increase in grinding stock removal caused by too large a diameter. Moreover, quality equalization is achieved by changing the crystal growth conditions in a crystal growth direction. However, the problem is that, when the diameter of a crystal deviates from a target, the amount of silicon melt in a crucible deviates from a target, causing a difference in quality.
On the other hand, in the weight method, it is common to adopt a method (a load cell method) as disclosed, for example, in JP-A-9-175893, the method for measuring the weight of a growing crystal by attaching a weighing machine called a load cell to an arm shaft. The load cell method is a method for calculating the diameter of a crystal based on the increment of weight per unit length. This method does not suffer an error, which would occur in the optical method, and, if an error of the load cell alone is kept track of, it is possible to measure a diameter. However, the maximum permissible weight of the load cell has to be set to a large value to cope with an increasing weight of a crystal, and this causes an increase in measurement error, or causes a reduction in sensitivity, making it impossible to calculate a diameter in a short time. When the diameter increases in a short time, the diameter has to be reduced to a target by increasing a crystal growth rate; however, the problem is that it is impossible to perform such control in a short time, resulting in production of a crystal having projections and depressions. Moreover, the problem is that, when the grown single crystal is a crystal having projections and depressions, variations in quality are exacerbated in these projections and depressions, or product failure is caused due to an insufficient diameter in a depression portion.